The invention relates to a high-pressure metal-halide lamp provided with a discharge vessel with an ionizable filling containing one or more metals among which Hg, a halide and a rare gas, which vessel comprises electrodes with a rod containing substantially W, the lamp, when in operation, maintaining a W-halide cycle in the discharge vessel.
A lamp of the type defined in the opening paragraph is known from U.S. Pat. No. 3,521,110. The known lamp, used as a light source of white light having a high luminous efficacy, has a discharge vessel with a wall of quartz glass. Other suitable wall material is ceramic. A ceramic wall is here understood to mean a refractive material such as monocrystalline metal oxide (for example, sapphire), polycrystalline metal oxide (for example, polycrystalline sintered aluminum oxide; yttrium aluminum garnet or yttrium oxide) and polycrystalline non-oxidic material (for example, aluminum nitride). The occurrence of the W halide cycle is the cause for an extension of the useful life of the lamp, because W evaporated from the electrode does not deposit on the entire surface of the wall of the discharge vessel. A condition for the occurrence of the W-halide cycle is the presence in the discharge vessel of a small amount of free oxygen when the lamp is in operation. A free oxygen source is generally contaminations occurring during the manufacture of the lamp and released therefrom when the lamp is in the operating state. It has also been established that oxygen is released from the wall of the discharge vessel under the influence of reactions to filling components of the discharge vessel.
A disadvantage of the known lamp is that the amount of oxygen available in the discharge vessel in the operating state of the lamp is uncontrollable. In the case of too small a concentration, it will hardly be possible to maintain the W halide cycle sufficiently during the operation of the lamp. In the case of too large a concentration, there will be, inter alia, corrosion of the W-rod electrode. In view of an accuracy of manufacture required for a proper operation of the lamp, the chance of too small a concentration of oxygen is ever more becoming a problem.
It has been proposed oxygen to dose in the filling, for example, in the form of oxyhalides such as, for example, niobium oxytrihalide (U.S. Pat. No. 4,672,267) or in the form of HgO (U.S. Pat. No. 3,720,855). A drawback of such dosings is that lamps manufactured thus burn unstably even when molecular stabilizers are used. A further drawback is that HgO is poisonous. A serious drawback of the use of Nb is that it has the tendency of dissolving in W and thus lowering the melting point of the W electrode, as a result of which a stronger evaporation of W will occur. The presence of free oxygen in a non-ignited lamp has a disadvantageous influence on the ignition of the lamp and is thus to be avoided.
It is an object of the invention to combat the described disadvantageous influence. For this purpose, the discharge vessel contains an oxygen dispenser.
The lamp according to the invention is advantageous in that oxygen is fed to the discharge vessel in a controlled manner during the operation of the lamp. An additional advantage is that dosaging during the life of the lamp becomes possible. In a first advantageous embodiment of the lamp according to the invention, the oxygen dispenser contains WO2. In a further advantageous embodiment of the lamp according to the invention, the oxygen dispenser contains CaO. Both WO2 and CaO have the advantage that, when O2 is supplied, elements are released which are commonly applied to discharge lamps, for example, as filling components. Preferably, an oxygen dispenser containing CaO is used in a lamp according to the invention with a ceramic discharge vessel. A surprising advantage of this lamp is that the Ca appears not only to maintain the W halide cycle, but also to deliver a spectral contribution both to red and in blue. Thus, a lamp can be manufactured which emits light that has a color temperature Tc of up to 3500 K and a value for the general color rendition index Ra of over 80.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.